Perfect once setup Rated 5 out of 5 stars

It takes awhile to setup correctly, and you have to have a little knowledge of what you're doing to do so. However, once it's setup, I've found this add-on to filter spam far better than Thunderbird's built-in filters... perfectly, even.

Just keep in mind that this works by filtering mail as it comes in BEFORE it even gets to thunderbird using a daemon that runs in the background. To configure it, you have to use its built-in web-interface (accessible from the add-on's configuration screen). The add-on itself facilitates the interface between thunderbird and the daemon, so sorting junk mail and keeping the filter up to date works just as easily as thunderbird's built-in filter... Brilliant.